Insightful post, G. I wonder just how much greater both Frazier and Norton would have been had they NOT been slow starters. I realize this is just more speculation. Had Joe still used his "smokin'"/FOTC offense/defense against Foreman, for instance, I think things would have been different.
You probably missed it but i posted previously that Norton's slow start was a tactical decision, which he stated himself. He knew Holmes had never fought 15 and actually thought he would slow down and he would get him in the late rounds. Remember Holmes stamina against top opposition was no-where near as proven as it was later, well after this fight really. He proved he could go 15 tough rounds against very good opposition. It was an error on Norton's part. That's not to say he would have won starting faster as the whole dynamic of the fight would change of course.
Sean O'Grady once said - "all of a sudden i couldn't take a punch". Most chins have an expiry date per punishment i would think and obviously some expire fast and some seem to last forever. some are out of the game before it ever happens. Mustafa Hamsho is a great example. He took immense punishment from Hagler in their first fight with aplomb excepting cuts and was stopped on his feet. Near the end he took some big blows but never looked like dropping. He was a beaten man but his chin would not allow him to be put down hence his handlers pulling things up. He was often cited as having the best chin in boxing. In the Hagler rematch however he was dropped and stopped without taking remotely as much punishment as he took in the first fight. Interestingly he soaked up some right hand from Lalonde quite well in a later bout before looking decidedly glass chinned against Rocchigiani at the end of his career.
